The subject matter of the present invention relates to a transfer unit for reliably transferring an explosive train from inside a pressure tight housing, through an externally pressurized bulkhead, to explosive devices disposed outside the housing which are exposed to the pressure and temperature of downhole borehole fluids.
One persistent problem which exists in wireline and tubing conveyed perforating is the lack of a reliable transfer of a strong detonation wave from one in-line explosive device to another, such as from a blasting cap to a detonating cord, from a booster to a detonating cord, or from detonating cord to booster. Transfer units are used to provide the detonation wave transfer. In typical prior art transfer units, the detonating cord abuts against the explosive interface of the booster or blasting cap. In these prior art transfer units, the transfer of a strong detonation wave is reliable provided the detonating cord abuts against the explosive interface. In fact, a reliable transfer can occur even though a small gap or space exists between the detonating cord and the explosive interface. The transfer is not reliable and may not occur, however, if the gap is large or if the end of the detonating cord is improperly prepared; this is particularly true if the transfer is from a detonating cord to a booster where shrinkage of the detonating cord has caused the inner core of the detonating cord to withdraw from the booster interface.
In addition, it is often necessary to interconnect two or more perforating guns to each other at a well site. When this is necessary, one must string a detonating cord, in series fashion, through each perforating gun in a tubing string. Since this task must be accomplished at the well site, it is a very time consuming task. It would be more convenient and far less time consuming for well site personnel if the detonating cord could be disposed in each perforating gun individually at its field shop, and adjacent perforating guns could be interconnected together at the well site by simply interconnecting their respective detonating cords. Furthermore, for perforating applications downhole, it is often desirable to initiate an explosive detonation train from inside a pressure-tight housing and to effect a transfer of the explosive train to explosive devices disposed outside the housing, which explosive devices are exposed to the pressure and temperature of downhole fluids. Since the explosive train is initiated by a detonator and electronics disposed inside the housing, the pressure-tight housing protects the detonator and electronics from the pressure and temperature of the downhole fluids. Conversely, it may also be necessary to transfer an explosive detonation train from a severe pressure and temperature environment disposed outside of the housing to the inside of the pressure tight housing in order to activate electrical or mechanical devices disposed inside the housing. Most typical detonation train transfer devices require the detonation train to transfer across a thick, pressure-tight transversely disposed metallic barrier or bulkhead, which bulkhead weakens the detonation train. As a result, the detonation train does not always transfer successfully across the bulkhead. When detonating from inside the pressure tight housing the problem is further aggravated by the pressure of the downhole wellbore fluid acting on the receptor explosive disposed outside of the housing. The fluid pressure makes the receptor explosive less sensitive to being detonated buy the donor explosive detonation train attempting to transfer across the bulkhead.